Among variable displacement compressors as disclosed in Japanese Laid-Open Patent Publication No. 2004-197679, there is a type having a displacement control valve the opening degree of which is controlled by detecting whether a refrigerant flow rate flowing through a passage provided within the compressor is proper. The opening degree of the displacement control valve is changed on the basis of a differential pressure between both sides of a restriction in a passage for the refrigerant in the compressor. In this displacement control valve, a force based on the differential pressure acts against an electromagnetic force generated by a current application to a solenoid within the displacement control valve via a valve body, and the opening degree of the valve is determined by arranging the valve body at a position where these two opposing forces are balanced.
The more the refrigerant flow rate increases, the higher the differential pressure between both sides of the restriction becomes. The differential pressure reflects the refrigerant flow rate, and the opening degree of the displacement control valve is increased when the differential pressure is increased. If the refrigerant flow rate becomes more than a proper flow rate, the opening degree of the displacement control valve is increased, and the amount of the refrigerant supplied to a crank chamber from a discharge chamber via a valve hole is increased. Accordingly, the pressure in the crank chamber is increased, the inclination angle of a swash plate is decreased, and the refrigerant flow rate is decreased to be converged into the proper flow rate. If the refrigerant flow rate becomes smaller than the proper flow rate, the opening degree becomes small, and the amount of the refrigerant supplied to the crank chamber from the discharge chamber via the valve hole is decreased. Accordingly, the pressure in the crank chamber is decreased, the inclination angle of the swash plate is increased, and the refrigerant flow rate is increased to be converged into the proper flow rate.
In the case that the compressor obtains a driving force from a vehicle engine, it is necessary to execute an output control of the engine to achieve an output capable of providing a necessary torque for driving the compressor. Since the refrigerant flow rate reflects the torque of the compressor, the torque of the compressor can be estimated by detecting the refrigerant flow rate. Although the differential pressure between both sides of the restriction reflects the refrigerant flow rate, the refrigerant flow rate is not actually detected. Accordingly, an estimation of the refrigerant flow rate (that is, the torque of the compressor) is executed on the basis of a magnitude of an electric current supplied to the solenoid of the displacement control valve.
At a time of starting the compressor, an operation control for setting the displacement to 100% is executed. However, since a liquid refrigerant in the crank chamber reserved during a stop of the operation of the compressor is vaporized with the start of the compressor, the pressure in the crank chamber becomes high, and the compressor maintains the operation while keeping the inclination angle of the swash plate small. A state in which the inclination angle of the swash plate is small corresponds to a state in which the torque of the compressor is small, that is, a state in which the refrigerant flow rate is small. On the other hand, the refrigerant flow rate estimated from the electric current supplied to the solenoid is large. Accordingly, even though the torque of the compressor is actually small, the operation of the vehicle engine is controlled on the assumption that the torque of the compressor is large. This causes an energy loss.
Accordingly, it is desirable to detect the refrigerant flow rate flowing within the variable displacement compressor by using a differential pressure type flow rate detector, for example, disclosed in Japanese Laid-Open Patent Publication Nos. 62-56820, 9-257534, and 2004-12394. When applying the flow rate detector to the compressor, the flow rate detector outputs an electric signal in correspondence to the differential pressure between both sides of the restriction provided within the passage for the refrigerant formed within the compressor.
Japanese Laid-Open Patent Publication No. 2004-12394 discloses a differential pressure type flow rate detector in which a first differential pressure chamber and a second differential pressure chamber are separated by a spool (a slidable partition body). In this detector, a high-pressure fluid is introduced to the first differential pressure chamber, and a low-pressure fluid is introduced to the second differential pressure chamber. The force based on the differential pressure between the pressure in the first differential pressure chamber and the pressure in the second differential pressure chamber acts against a spring force of a spring urging the spool toward the first differential pressure chamber from the second differential pressure chamber. A detection body coupled to the spool is arranged at a position at which the differential pressure and the spring force are balanced, and the electric signal according to the detection body is output.
In the flow rate detector disclosed in Japanese Laid-Open Patent Publication No. 2004-12394, the differential pressure between the pressure in the first differential pressure chamber and the pressure in the second differential pressure chamber is generated by a pipe orifice provided in the middle of the passage connecting the first differential pressure chamber and the second differential pressure chamber. However, the installation of the pipe orifice causes an increase of a flow resistance of the refrigerant, and is not preferable. Further, since the spool (the partition body) comes into slidable contact with a peripheral wall surface of the spool chamber, the sliding parts wear.